secondary air system

I have a 2001 Chevy S-10 blazer V-6. The service engin soon light is on and the computer says it is the secondary air system. Can someone tell me what this is and what to check/replace? The truck has 70000 miles on it and it is running normal. Gas mileage is a bit lower than normal but the running is fine, at least as far as one can tell without further testing.

What code is it?